All mech Bays have full transport, repair, and drop capabilities. The Excalibur's inability to drop mechs is purely fluff, and is not supported by any rules. A GM might choose to incorporate that flaw into a game they are running, but under all published rules, Excaliburs can drop mechs just as easily as any other DropShip with mech bays.
(TRO 3057 has a lot of examples of unsupported fluff. Best to ignore them and only look at the actual stats until such time as we can get quirks out for them.)